Energy recovery from waste gases in the chemical or petrochemical industry has been increasingly popular. Process gases, whose energy content is sufficient to supply between 30% and 100% of the compression work (driving power for the necessary compressors) for the chemical process are released from the chemical process in such processes as FCC (Fluid Catalytic Cracking), PTA (Terephthalic Acid Production), in nitric acid production, and other processes.
Gas expansion turbines or expanders are usually used to recover this energy.
Waste gas flows through these gas expansion turbines before it is discharged into the atmosphere. The gas expansion turbines are frequently arranged on a single machine shaft together with compressors for compressing the process gases, so that they can directly drive the compressor.
Gas expansion turbines that drive only an electric generator are also used in other cases of application, especially when an existing plant is to be expanded by an energy recovery unit.
The typical output classes for such gas expansion turbines are 6-20 MW; however, these units are also built for outputs of up to 60 MW.
To maintain the process pressure at a constant value even in the case of variable amounts of waste gas, there are regulating valves at the inlet of the gas expansion turbine. These regulating valves throttle when the amount of waste gas decreases, and they open when the amount of waste gas increases.
In the case of overload, bypass valves arranged in parallel to the gas expansion turbine are able to bypass part of the waste gas past the gas expansion turbine directly into the atmosphere. These bypass valves are together made so large that they are also able to bypass the total amount of waste gas into the atmosphere when the expander is switched off.
Gas expansion turbines or expanders belong in the class of the rotating machines and require an emergency switchoff system. Overspeed is one of the severe incidents that can occur with a gas expansion turbine.
To prevent an overspeed, the inlet valves of the gas expansion turbine must close within 0.6 to 2 sec. This case of overspeed is especially critical in machine constellations, arrangements and configurations in which a gas expansion turbine drives an electric generator only.
Should the generator be suddenly disconnected from the power line, e.g., due to a disturbance in the electric part of the generator, the entire power of the gas expansion turbine is available for accelerating the machine set.
It can be ensured that the increase in speed remains limited to values below 10% of the rated speed only if the gas throughput is completely interrupted after 0.6 sec.
The case of overspeed may also occur in gas expansion turbines which are mounted on one shaft together with compressors. Quick-acting inlet valves are necessary in this case as well. However, the closing time may be prolonged to up to 2 sec in this case.
Energy recovery units are to be designed such that they do not compromise the chemical process. This also applies to an emergency switchoff. Protective and regulating means are to be provided, which shall ensure in the case of an emergency switchoff that the process pressure is not subject to any unacceptable changes.
In the case of gas expansion turbines, this means that the bypass valves to the gas expansion turbine (bypass valves) must open in the case of an emergency switchoff so quickly that they allow the total amount of waste gas to flow through.
The differential pressure that becomes established over the bypass valves must be exactly equal to the differential pressure that acted before the emergency switchoff over the expansion turbine, taking additionally into account the throttling action of the inlet valves.
Pressure regulations of gas expansion turbines which comprise a regulation of regenerator outlet pressures in FCC (Fluid Catalytic Cracking) plants have been known.
Three valves are moved in the "split range" in this pressure regulation. As the output signal of the process pressure regulator increases, a valve is first opened at the inlet of the expansion turbine. Once this is fully open, a small bypass valve begins to open. Once this small valve is fully open as well, a large valve opens, so that all three valves are open at full output signal of the process pressure regulator.
A large (second) bypass valve and a small (first) bypass valve are used because the volume flows can be better regulated with a small valve than with one large valve. Only a small partial amount is usually controlled in the bypass. This small amount can be better regulated with a small valve than with a large one.
Each of the three valves is additionally energized by safety controls. These safety controls act independently from the regulation and move the corresponding valve into a predetermined, safe end position when a control responds. This is the closed position for the inlet valve of the expansion turbine, and the open position for the first or second bypass valve.
If a load shedding of the generator or another change in load occurs, the inlet valve of the expansion turbine, which is controlled by the safety control, is closed. The pressure in the process increases as a consequence. This is recognized by the process pressure regulator and increases its output signal. The first (small) bypass valve opens first via a "split range" control, and the second (large) bypass valve will be opened wide enough for the process pressure to reach its desired value again.
However, this regulation process has the drawback that considerable variations occur in the process pressure. In particular, the pressure increases markedly immediately after the load shedding, before the process pressure regulator can intervene and can stop the increase in pressure by opening the bypass valves.